Let's do some necromancy! Getting a castle as a vassal boils down to being well liked by your peers(voting) and is increased by leading the successful siege.
Now for the interesting part - conquering a castle as a private knight! Most importantly to keep in mind you don't have to win - get in, kill some defenders, go home to raise another army.
One successful tactic is achieving "arrow superiority". Despite the castle defenders having clear advantage in archery, if your whole army is only archers, most of the defenders will just stand and wait while you kill their archers off. It's embarrassing.
Another idea - again either with archers or a shooting main character - is maneuvering so that your ranged units(could be just you) see only a few of the unfriendly shooters. Castles in hilly areas are susceptible.
Something I'm not a fan of but does work: huscarls are by far the best infantry. Just order 100 of them to die on those walls and pop a cold one.
The comando way is to somehow rush past the defenders and slaughter whoever you can while everyone is concentrated on the breach.
Or wait for a castle that just changed hands. Those usually have as few as 30 basic troops but are in hot zones and you lose them right away. Sometimes one faction would capture a secluded castle of another - this is a priceless opportunity!
Once you have a couple of loyal lords you can gather everyone and take any castle in one go.
A word on keeping castles. You can't*. Just wait for the huge army to move on with its business and recapture it. At some point they get fed up and offer peace. For winnable fights keep in mind fortification are approximately x2 force multiplier.
* - context - you are not yet an established kingdom with cities and distinct armies